1. Technical Field of the Invention
This invention relates generally to wireless communication systems and, more particularly, to radio frequency transmitters used within such systems.
2. Description of the Related Art
As is known, use of frequencies for wireless communications is partitioned into frequency bands by government agencies. For instance, the Federal Communications Commission (FCC) defines, for the United States, frequency bands for specific uses and for which an FCC license is required (e.g., radio transmissions, television transmissions, etc.) and also defines frequency bands that are unlicensed and, as such, can be used for a variety of applications. For instance, the FCC has defined several frequency bands in the radio frequency spectrum as being unlicensed. Such unlicensed frequency bands include 902–928 MHz, 2.4–2.483 GHz and 5.75–5.85 GHz, which are collectively referred to as the ISM (Industrial Scientific Medical) band. Currently, the ISM band is used for in-building and system applications (e.g., bar code readers), industrial microwave ovens, wireless patient monitors, and wireless local area networks (WLAN).
As is also known, there are standard bodies that define standards for WLAN equipment within the ISM band. Such standards include, but are not limited to, Bluetooth, IEEE 802.11(a), IEEE 802.11(b), and IEEE 802.11(g). The IEEE 802.11(g) standard provides wireless LAN operation specifications in the 2.4–2.482 GHz band. Specification applies to both transmitters and receivers and defines data rates, modulation schemes, transmitter architectures, receiver architectures, etc. In general, the specified modulation schemes are based on Orthogonal Frequency Division Multiplexing (OFDM) which, for 802.11(g) divides the 2.4–2.482 GHz band into a plurality of channels. Further, as specified, the channels at the boundaries of the frequency band (e.g., channels 1 and 11 that are centered at 2.412 GHz and 2.462 GHz, respectively) are preferred channels of use.
As is further known, the FCC has defined exclusion frequency bands around the 2.4–2.482 GHz frequency band. One exclusion frequency band begins at 2.390 GHz and includes lower frequencies and the other exclusion frequency band begins at 2.4835 GHz and includes higher frequencies. An issue arises when fabricating a transmitter in accordance with the architecture defined within the 802.11(g) specification, in that, for channels 1 and 11, the transmit power levels in the exclusion frequency bands are too great, which violates the FCC restrictions on use of the exclusion frequency bands.
Therefore, a need exists for a transmitter that provides desired power levels within prescribed frequency bands of operation and does not violate transmit power levels in non-prescribed frequency bands.